Chemical pulping is a process whereby wood chips, wood shavings, and/or sawdust are heated at elevated temperatures in an aqueous acid or alkaline solution, also known as white liquor or cooking liquor, in order to remove enough lignin so that the cellulose fibers can be readily separated from one another. Typically, the process is carried out by heating a mixture of wood chips and cooking liquor in a large pressure vessel called a digester. The cooking temperature is usually in the 170-175.degree. C. range with a corresponding cooking time of at least 90 minutes. The cooked chips are discharged or blown from the digester under pressure, the mechanical force of which breaks up the wood chips into individual fibers, producing the pulp. The pulp from the digester contains fiber and exhausted liquor which is black in color. The black liquor is washed from the pulp which is then screened to remove uncooked chips and other large fragments and sent on for further processing.
The efficiency of the pulping process is reflected in the degree of delignification which depends upon the extent of the penetration of the cooking liquor and the uniformity of the distribution of the liquor within the chips. Inadequate impregnation usually results in a high level of screen rejects and low pulp yield. The current trends in research and development of the pulping industry are leading towards the use of digester aids. Digester aids are materials that are added to the white liquor to increase the yield and rate. To be most efficient, these digester aids must be soluble and stable under the pulping conditions.
Anthraquinone is an example of a compound that is widely employed as a digester aid because of its relatively low cost and lack of interference with downstream paper making operations.
Unfortunately, the known digester aids are not completely satisfactory, for example, for environmental considerations in certain cases or for lack of adequate penetration and extraction of undesirable organic components in other cases. Prior to the present invention, there existed no known system which enhanced the efficiency of the pulp digestion through enhanced removal of lignin and other extractives to desired levels.